1. Field of the Invention
The present invention relates to a magnetic resonance diagnostic apparatus having a safety management function based on SARs (Specific Absorption Rate).
2. Description of the Related Art
A magnetic resonance diagnostic apparatus (to be referred to as an MRI apparatus hereinafter) is designed to apply a high-frequency magnetic field for causing magnetic resonance to an imaging region by using a transmission high-frequency coil. The resonance frequency of a high-frequency magnetic field is proportional to the static field strength of the MRI apparatus. If, for example, the apparatus has a static field strength of 1.5 T, the resonance frequency is 63.8 MHz. It is known that a high frequency in this frequency region heats a subject to be examined and raises his/her body temperature. In general, concerning the human body, an increase in body core temperature is more problematic in terms of safety than an increase in skin temperature. When an increase in the body core temperature of the human body exceeds 1° C., a trouble may occur. For this reason, the upper limit value of temperature rise in terms of safety is set to 1° C., and the upper limit value of SAR (Specific Absorption Rate), which is the high-frequency output absorbed by 1-kg tissue, is set in IEC standards (IEC 60601-2-33 Second Edition: Particular requirements for the safety of magnetic resonance equipment for medical diagnosis) or JIS standards (JIS Z4951 “Particular requirements for the safety of magnetic resonance equipment for medical diagnosis”). For example, in the normal operating mode based on IEC standards, the upper limit of SAR value with respect to the whole body is 2 W/kg, and the upper limit value of SAR value with respect to the head is 3.2 W/kg.
For example, before imaging operation, an operator inputs the weight (kg) of a patient as part of patient information such as the patient name, age, and sex through the console. The operator positions a subject P to be examined in a magnetic gantry 2 of an MRI apparatus, and applies high-frequency waves from a high-frequency amplifier 3 to the subject P through a transmission high-frequency coil 4 to set high-frequency conditions for obtaining an MR signal optimal for an imaging region of the subject P. The operator then performs imaging operation using the conditions. Since the difference between the high-frequency output which was applied when the high-frequency conditions were set and the high-frequency output measured under the condition of no subject is the high-frequency output (W) absorbed by the subject, the quotient of the output value by the patient's weight input in advance is a whole body SAR value (W/kg). The MRI apparatus calculates a whole body SAR value before imaging operation, and displays it on the screen of a console (1). If the obtained SAR value exceeds the upper limit value in the above safety standards, a corresponding warning is displayed on the screen of the console (1). The operator then changes the imaging conditions or switches the current mode to an upper-level operating mode.
In a general example of changing the imaging conditions, the operator may make setting again to decrease the initially set number of multi-slices. In this case, the necessary imaging region cannot be covered by one set of multi-slices, and hence two or more imaging operations are required, resulting in an increase in overall MRI examination time. With regard to the switching of operation modes, a first level controlled operating mode and secondary level controlled operating mode are defined as well as the normal operation mode by the safety standards. Medical determination on this switching operation is performed in consideration of the relationship between potential risks and merits for the patient. For example, in the primary level controlled operating mode, the upper limit value of whole body SAR value is 4 W/kg.
The axial length of the transmission high-frequency coil of a conventional MRI apparatus is about 50 to 60 cm. As an SAR value for adults, aside from children, the SAR value obtained by dividing a high-frequency output by the weight of the whole body including a portion to which no high-frequency output is applied does not necessarily coincide with the SAR value of the partial body to which a high-frequency output is actually applied. On the other hand, as high-speed imaging methods and high-field MRI apparatuses have been developed, pulse sequences for obtaining MR images which ensure higher diagnostic performance require higher SAR values.
According to IEC standards (IEC 60601-2-33, 2nd Edition) revised in 2002, the following is defined as a partial body SAR:
In case of normal operation mode:partial body SAR=10(W/kg)−(8(W/kg)×(weight of part to which high-frequency waves are applied)/(weight of patient))An SAR value in the range of 2 W/kg to 10 W/kg is set as an upper limit in accordance with the ratio between the weight of a part of a patient to which high-frequency waves are applied and the weight of the patient.
A conventional method of performing management on the basis of only whole body SARs cannot cope with definition of this partial body SAR, and hence cannot provide diagnosis information with higher accuracy. Furthermore, even if the transmission high-frequency coil is reduced in length to be directed to a specific region such as the heart so as to limit the area to which high-frequency waves are to be applied, a conventional method, e.g., the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-317287, cannot cope with this.